Paraxylene recovery

ABSTRACT

IN AN ISOMERIZATION ZONE, C8 AROMATICS ARE CONVERTED INTO XYLENE ISOMERS AND NAPHTHENES. CONVENTIONAL ISOMERIZATION TECHNIQUES ARE USED TO PRODUCE THE XYLENES AND NAPHTHENES, AND CONVENTIONAL FRACTIONAL CRYSTALLIZATION TECHNIQUES ARE THEN USED TO SEPARATE PARAXYLENE FROM OTHER ISOMERS OF XYLENES. THE DISTINGUISHING FEATURE OF MY INVENTION IS THE SEPARATION OF THE XYLENES FROM THE NAPH-   THENES BEFORE CRYSTALLIZATION AND THEN RECYCLING THE NAPHTHENES TO THE ISOMERIZATION ZONE.

Fb 9, 1971 R.KJ. HENGSTEBECK 3,562,344

PARAXYLENE RECOVERY Filed May 2l, 1969 AT TOR/VE Y United States Patent Oice 3,562,344 Patented Feb. 9, 1971 3,562,344 PARAXYLENE RECOVERY Robert J. Hengstebeck, Valparaiso, Ind., assignor to Standard Oil Company, Chicago, Ill., a corporation of Indiana Filed May 21, 1969, Ser. No. 826,637 Int. Cl. C07c .I5/08 U.S. Cl. 261)-668 3 Claims ABSTRACT OF THE DISCLOSURE In an isomerization zone, C3 aromatics are converted into xylene isomers and naphthenes. Conventional isomerization techniques are used to produce the xylenes and naphthenes, and conventional fractional crystallization techniques are then used to separate paraxylene from other isomers of xylenes. The distinguishing feature of my invention is the separation of the xylenes from the naphthenes before crystallization and then recycling the naphthenes to the isomerization zone.

BACKGROUND OF THE INVENTION According to well known practice, a feed rich in C8 aromatics and hydrogen is irst passed through an isomerization zone maintained at suitable isomerization conditions and containing a suitable hydrogenation-dehydrogenation catalyst. Typical conditions and catalysts are illustrated in U.S. Pat. No. 2,976,332 and No. 3,180,839. In the isomerization zone, Cg-rich aromatics are converted to a mix of xylene isomers and naphthenes, and then the mix is withdrawn from the isomerization zone.

This mix could be fed directly into a fractional crystallization unit where the different isomers of xylenes would be separated from each other using conventional techniques and standard refrigeration equipment. If this is done, however, the load on the refrigeration equipment would be high because of the presence of naphthenes in the mix. The xylenes could be rst separated from the naphthenes and then fed to the fractional crystallization unit. If this is done the load on the refrigeration equipment is reduced, but yields of paraxylene, the most valuable isomer product, are also reduced.

DESCRIPTION OF THE INVENTION I have invented a process for increasing the yield of paraxylene from a mixture of xylene isomers without placing an undue burden on the refrigeration equipment normally used in separating the paraxylene from other xylene isomers. According to my process, the naphthenes are iirst separated from the xylenes and light byproducts and then recycled directly to the isomerization zone. When this is done, the yield of paraxylene will range between about 74-76 lbs/100 lbs. of xylene feed, as contrasted to a yield of 71 lbs. of paraxylene/.IOO lbs. of xylene feed when naphthenes are separated from the xylenes but not recycled to the isomerization zone.

Separation of the light byproducts, naphthenes and xylenes may be achieved by fractional distillation, with the naphthene fraction boiling between the range of about 235 and about 265 F. Paraxylene recovery is achieved by conventional fractional crystallization. Any unseparated xylenes are recycled to the isomerization zone.

DESCRIPTION OF A PREFERRED EMBODIMENT The ligure, which is a schematic ilow digram illustrating my process, shows C8 aromatics from source l0 being mixed with hydrogen from source 12 and then fed via line 14 to isomerization zone 16. The eliluent from isomerization zone 16, which is a mixture of light byproducts, naphthenes and C8 aromatics, flows Via line 18 into stabilizing tower 20. The naphthene-rich fraction boiling between 23S-265 F. is drawn off through line 22 and recycled via this line and line 14 to zone 16. This maintains the optimum or near optimum equilibrium concentration of naphthene in zone 16, so that most of the C8 aromatics entering zone 16 are converted to xylenes. Optimum naphthene concentration is dependent on pressure, temperature, and catalyst in zone 16, and 'will vary as these parameters vary.

Light byproducts boiling below 231 F. ow via line 23 out of the top of tower 20, and mixed xylenes ilow via line 24 into xylene recovery tower 26. C9 and higher hydrocarbons ow via line 28 from the bottom of tower 26, and the mixed xylenes escape from the top of tower 26 and flow via line 30 into conventional fractional crystallization zone 32. In crystallization zone 32, paraxylene is preferentially recovered and drawn off via line 35. Unseparated xylenes ow from zone 32 via line 36 and line 14 into isomerization zone 16.

I claim:

1. In a process for producing paraxylene wherein a feed rich in C8 aromatics and hydrogen are passed through an isomerization zone maintained at suitable isomerization conditions and containing a suitable hydrogenationdehydrogenation catalyst, whereby a substantially equilibrated mix of xylenes and naphthenes form in said zone, and then said mix of xylenes and naphthenes are lwithdrawn from the isomerization zone and the xylenes are passed through a fractional crystallization zone to separate paraxylene from other isomers of xylenes, the improvement comprising separating the naphthenes from the xylenes before said xylenes are passed into the fractional crystallization zone and recycling said naphthenes to the isomerization zone.

2. The process defined by claim 1 wherein separation of the xylene and naphthenes is achieved by fractional distillation, with the naphthene fraction boiling between about 235 and about 265 F.

3. The process dened in claim 1 wherein unseparated xylenes are withdrawn from the fractional crystallization zone and recycled to the isomerization zone.

References Cited UNITED STATES PATENTS 2,322,482 6/ 1943 Stahly et al. 260-683.59X 2,403,785 7/ 1946 Britton et al 260-671 3,159,687 12/1964 Lehman 260--668A 3,268,608 8/ 1966 De Rossett 260-674 3,274,278 9/ 1966 Kapur et al. 260-671 3,381,048 4/ 1968 Love et al 260--668 CURTIS R. DAVIS, Primary Examiner U.S. Cl. X.R.

Notice of Adverse Decisions in Interferences In Interference No. 97,946 involving Patent No. 3,562,344, R. J. Hengstebeck, PARAXYLENE RECOVERY, nal judgment. adverse to the patentee was rendered May 29, 1973, as to claims 1, 2 and 3.

[Ocial Gazette September 4, 1.973.] 

